The present invention relates to analog optical transmission systems, and more particularly to an apparatus and method for predistorting a modulation signal to provide linear operation of an external optical modulator.
Optical transmission systems are currently being implemented for use in various communication applications. For example, telephone systems are now in use that utilize optical fiber technology to transmit voice and data signals over long distances. Similarly, cable television networks are now available where an optical fiber technology is used for the transmission of both analog and digital signals.
Prior to the implementation of optical transmission networks, cable television programs were carried as radio frequency ("RF") signals over electrical coaxial cables. In such systems, it is usually necessary to transmit signals over long distances. Since the strength of transmitted signals decreases in proportion to the length of the cable over which the signals are transmitted, amplification at repeated intervals along the cable is necessary to maintain adequate signal strength. The electronic amplifiers used for this purpose inherently distort the signals as they are being amplified. Harmonic distortion, in particular, results from nonlinearities in the amplifier that cause harmonics of the input frequencies to appear in the output. Other components in the communication network can similarly introduce distortion components.
One solution to the distortion problem in coaxial cable RF communication systems has been to use "feedforward" amplifiers to amplify the signals at repeated intervals along the cable. Feedforward amplifiers are described, for example, in Blumenkranz U.S. Pat. No. 4,472,725 entitled "LC Delay Line for Feedforward Amplifier", Tarbutton et al U.S. Pat. No. 4,617,522 entitled "Temperature Regulated Feedforward Amplifier", Mannerstrom U.S. Pat. No. 4,625,178 entitled "Feedforward Gain Block with Mirror Image Error Amplifier", and Blumenkranz et al U.S. Pat. No. 4,792,751 entitled "Apparatus for Field Testing the Performance of a Feedforward Amplifier".
In the operation of a feedforward amplifier, a signal component representative of the distortion introduced by the amplifier is extracted from the amplified signal. That component is then typically phase inverted and the phase inverted distortion component is combined with the amplified signals. The phase inverted component cancels the distortion component in the amplified signal, leaving a relatively distortion free amplified signal for transmission along the cable.
Optical transmission systems provide a new problem in coping with distortion. In particular, harmonic distortion is a major limitation in analog amplitude modulated optical communication transmission systems. In order to maintain a desired carrier-to-noise ratio of at least 58 dB in a cable television communication system, for example, it is preferable to use external modulators to modulate an optical carrier with the communication signal. However, currently available external modulators, such as Mach Zehnder intensity modulators are nonlinear, due to the phase to intensity conversion process. Nonlinearities of the modulator result in odd order harmonic distortions, of which the third order components are most significant.
It would be advantageous to provide an apparatus and method for reducing harmonic distortion in an analog optical transmission system using an external modulator. Such an apparatus and method would have particular application in an AM cable television supertrunk distribution system. It would be further advantageous to provide such an apparatus and method that is economical, readily manufacturable, and reliable. The present invention provides such an apparatus and method.